U-Haul trailers are always heavier than you expect them to be. They're built to take the abuse of people who don't know how to trailer.
Even a bare bones 5 X 9 open utility trailer is over 1200 pounds.
The OP didn't indicate which transmission he had, but towing at the max limit like that would make the convertor lock-up clutch a high wear point in the GM 5L40-e, possibly trashing the fluid and filter with debris.